This invention relates to a video disc on which video and audio information signals are recorded and a playback system for the video disc.
There is generally known video disc playback systems which reproduce FM signals including a video information signal (including luminance signal and chrominance signal components) and an audio information signal from a rotating video disc which has a spiral information track formed of pits corresponding to the FM signals, by bringing a stylus into contact with the information track to detect changes in capacitance between an electrode formed on one surface of the stylus and an electrode formed on the video disc which are caused by the existence of the pits. The video and audio information signals are separately extracted from the reproduced FM signals by the frequency separation method, and are respectively subjected to additional demodulation.
There are grooved and ungrooved type video discs. In a grooved type video disc, a groove is formed along a track, and pits are defined at the bottom of the groove. In the case of such grooved type disc, therefore, the stylus traces the groove, so that tracking servo is not required. Since the stylus traces the groove, however, reproduction from the video disc is limited to a normal playback mode; it is difficult to achieve picture reproduction in special playback modes, such as quick-motion playback, slow-motion playback, still picture playback, etc.
In order to enable picture reproduction in the special playback modes, there has been developed such an ungrooved type video disc as shown in FIG. 1. In FIG. 1, numeral 11 designates a disc base formed of synthetic resin on which pits 12 are formed along a recording track. Signal information is recorded as changes of the length of the pits along the longitudinal direction of the track and the space between the pits. A metal coating layer 13 is formed on the disc base with the pits thereon, and a dielectric layer 14 is formed on the metal coating layer 13. The dielectric layer 14 is coated with a smoothing agent 15 so that the surface of the video disc may be smooth. Numeral 16 denotes a sapphire or diamond stylus. An electrode 17 is formed by evaporation on a surface of the stylus 16 at right angles to the longitudinal direction of the recording track and parallel to the traverse direction thereof. The capacitance between the electrode 17 on the stylus 16 and the electrode 13 on the disc base changes according to the recording information as the video disc rotates. If the disc 11 is formed of a conducting plastic material, the metal coating layer 13 and dielectric layer 14 may be removed.
In such ungrooved video disc, although picture reproduction in the special modes may be easily performed due to its flat or even surface, a tracking servo system is essential to the accurate tracing of the recording track. For tracking servo, there has been developed a video disc in which tracking information pits are formed between information tracks for the main signals, the video and audio signals. Such video disc would, however, be too complicated to be manufactured with ease, and require sophisticated signal processing circuits for tracking servo.
According to the prior art video disc signal recording system, video signal information (including luminance and chrominance signal components) and audio signal information are simultaneously recorded on a common track. An FM wave bearing the audio signal information is recorded at an attenuated level and with a different frequency band, as compared with an FM wave bearing the video signal information. In this case, a narrower frequency band is allocated to the audio signal information as compared with the case of television or FM radio broadcasting. The low recording level of the audio signal FM waves for the prevention of interference between the video and audio information signals constitutes a hindrance to reproduction of high-quality audio signals.
The chrominance signal component is non-interferringly superposed on the luminance signal component, and then recorded on the video disc in the form of an FM signal. In order to reproduce a color picture on a color picture tube, the luminance and chrominance signal components are separated from each other after FM demodulation, the chrominance signal component is frequency-converted, and these two signal components are recomposed into a form suited for the reproduction on the color picture tube. The separation between the chrominance and luminance signal components requires a complicated and expensive comb filter.